css-notes/9. ES6

ES6 - The Future Is Now! (Sort of)

Lesson Objectives

1. What is ES6
2. strict mode
3. understanding MDN
4. polyfills and shims
5. babel/transpiling
6. IFFE
7. Block level scoping with let
8. const
9. arrow functions
10. Classes
11. for...of
12. default value setting
13. Array.isArray()
14. argument object
15. spread and rest operators
16. Template Literals
17. Object literal extensions
18. Destructuring
19. swap (desconstucturing method)

What is ES6

ECMAScript is the standard version of JavaScript. ECMA stands for European Computer Manufacturers Association. ES5 became standard in 2009. ES6 (or ES2015), became standard in 2015. But just because it is made standard, it doesn't mean that all browsers/environments are ready/compliant.

Strict Mode

Adding 'use strict' to the top of your code, (see Babel screenshot) will let you opt in to some newer features of JS! More info

understanding MDN

• Lets you know which methods/syntaxes are standard, experimental, depreciated, when things were added and more including shims/polyfills.

  • forEach on MDN

• Quickly see if a method is standard[no notation], experimental[flask], depreciated (removed from standards, may be removed)[thumbs down], or obsolete (likely removed altogether in newer environments)[trash can]

  

• See more information about when the method was defined implemented:

  

Polyfills/Shims

Some people call them polyfills, some call them shims; either way: It is code that is a fallback that provides the functionality when the expected functionality is not available (because old browser, etc. ). Many entries in MDN have polyfills. This is also a great way to look at how the ECMAScript standard code is written (a lot of examples are directly from the official ECMAScript docs)

An example Polyfill:

babel/transpiling

Now! But it depends where. Node.js supports most/nearly all new things. Chrome, Firefox, Edge, and Safari have been keeping up well. But new features are being added and tested. To be sure that code will run across many platforms (never forget the people who still use IE8), many people have come to rely on transpilers/compilers. Transpilers/Compilers take 'dialects' of standard code (incuding ES6, coffeescript, typescript and more) and convert the code for you. A popular one for ES6 is Babel, they have both repl (Read, Evaluate, Print, Loop) and an npm module/gem.

You can also run babel from within the command line, but most often it's used a build tool like webpack. First install:

npm init
npm install babel-cli
npm install babel-preset-es2015

Now in your package.json add this into "scripts":

"scripts": {
  "build": "babel --presets es2015 test.js -o compiled.js"
},

Now create a test.js file:

touch test.js

In it, write some ES6:

class Car {
    run(){
    }
}

Now run npm run build and take a look at compiled.js:

"use strict";

var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Car = function () {
	function Car() {
		_classCallCheck(this, Car);
	}

	_createClass(Car, [{
		key: "run",
		value: function run() {}
	}]);

	return Car;
}();

That's a spicy meatball!

If you want, you can adjust package.json to re-compile every time you make a change:

"scripts": {
  "build": "babel --watch --presets es2015 test.js -o compiled.js"
},

IIFE

Normally, variable declarations, are "hoisted" up to the top of whatever function they are declared in (or global if no function exists). This can lead to some weird moments:

var a = 2;

if(true){
    var a = 4;
}

console.log(a); //in many programming languages, this would be 2

and a famous interview question:

for(var i=0; i<5; i++){
    setTimeout(function(){
        console.log(i) //you'd expect 0,1,2,3,4
    },500 * i);
}

IIFE - Immediately Invoked Function Expression - often used to create a block of scope. This is not an ES6 thing, but rather an example of an old way to get block scoping.

var a = 2;

(function IIFE(){
    var a = 4;
    console.log ('Inside the IFFE, the value of a is', a ); //"Inside the IFFE, the value of a is 4"
})();

console.log ( 'Outside the IFFE, the value of a is', a ); //"Outside the IFFE, the value of a is 2"

And now the loop with setTimeout:

for(var i=0; i<5; i++){
  (function(i){
    setTimeout(function(){
      console.log(i)
    },500 * i);
  })(i)
}

Block Scoping with let

A new way to get block level scoping is with the keyword let, and the use of {} as in any {}, not just tied to a function! Note: let is NOT meant to completely replace var!

var a = 2

{
    let a = 4
    console.log( 'the value of b inside the `{}` is', a); //"block level scope"
}

console.log ('the value of b outside of the `{}` is', a) //"global scope"

And now the loop with setTimeout:

for(let i=0; i<5; i++){
    setTimeout(function(){
        console.log(i) //you'd expect 0,1,2,3,4
    },500 * i);
}

console.log(i); //notice i is not defined outside of the loop

Const

Another new way to declare a variable. This will block the value from being reassigned.

const pi = 3.14;
 pi = "pie" //TypeError: Assignment to constant variable.

const whatsForLunch = [ "Nuts", "Coffee" ]
whatsForLunch.push( "Yogurt" ) //you can still call object methods that alter the variable
console.log( whatsForLunch)
whatsForLunch.shift();  //you can still call object methods that alter the variable
console.log( whatsForLunch )
whatsForLunch = ["salad"]; //TypeError: Assignment to constant variable.

Arrow Functions

A new way to write a function expression (arrow functions are always anonymous functions), with fun shortcuts:

Shorthands

Basic:

var basicES5 = function(){
    return 'oh hai';
}

//take out the keyword 'function' and add in '=>'
var basicES6 = () => {
    return 'oh hai'
}

If the body of the function is just one expression that gets returned, we can remove the {} and ES6 knows to return whatever comes after the =>

var basicES6 = () => 'oh hai'

Multiple arguments:

//ES5 - Multiple arguments
var sumES5 = function (a, b){
    return a+b;
}

console.log('ES5 sum:',sumES5(5,5));

//ES6 - Multiple arguments
var sumES6 = ( a, b ) => a + b;

console.log('ES6 sum:', sumES6(6,6));

If we only have one argument to the function, we can leave out the ()

//ES5 - one argument
var squareES5 = function (c){
    return c*c;
}
console.log('ES5 square:', squareES5(5));

//ES6 - one argument
var squareES6 = c => c*c
console.log('ES6 square:',squareES6(6));

Binding

Here is an example of the arrow function binding this

// Create constructor function, inputs name, has default friend values
function Person ( name , friends = ["Charlie", "Dennis", "Ron", "Sweet Dee", "Frank"]){
    this.name = name;
    this.friends = friends;

    //Add four methods:

    // The first, `secret Admirer`, `this.name` is undefined in the forEach function
    this.secretAdmirer = function (){
        this.friends.forEach(function ( f ){
            console.log( this.name + " sends flowers to " + f );
        });
    }

    //The second one is the way we got around the issue of `this` - which was to set the desired `this` to a variable called `that` or `self` or something similar:

    this.giveOldSchoolLove = function (){
        var self = this;
        this.friends.forEach(function ( f ){
            console.log( self.name + " likes " + f );
        });
    }

    // we could also use .bind()
    this.giveBindingAffection = function (){
        this.friends.forEach(function ( f ){
            console.log( this.name + " makes friendship bracelets for " + f )
        }.bind(this));
    }

    //Finally, by using the arrow function, `this` is already bound:

    this.giveNewSchoolLove = function (){
        this.friends.forEach(f => console.log( this.name + " hearts " + f ));
    }
}

//See examples
k = new Person ( "Matt" );
console.log( 'Secret Admirer:' );
k.secretAdmirer();
console.log( 'Show old school love:' );
k.giveOldSchoolLove();
console.log( 'Show new school love:' );
k.giveNewSchoolLove();

Support for Classes

class Cat {
    constructor(name) {
        this.name = name;
    }
    makesNoises() {
        return (this.name + 'meows');
    }
    purrs(){
        return (this.name + 'purrs');
    }
}

class Lion extends Cat {
    makesNoises() {
        return (this.name + 'roars!');
    }
    eatHuman(){
        return 'yum';
    }
}

var myCat = new Cat("Kitty");
console.log(myCat.makesNoises());
console.log(myCat.purrs());

var myLion = new Lion("Simba");
console.log(myLion.makesNoises());
console.log(myLion.purrs());
console.log(myLion.eatHuman());

for...of

Until now, the best way to loop through an array was either:

var array = [1,4,6];
for(var i =0; i < array.length; i++){
    console.log(array[i]);
}

Or:

var array = [1,4,6];
array.forEach(function(element){
    console.log(element);
});

But now we can do:

var array = [1,4,6];
for(let element of array){
    console.log(element);
}

Default values

When creating a constructor and you wanted a default value, you previously had to write something like this:

function Beverage( type ){
    this.type = type || "water";
}
var breakfast = new Beverage();
var breakfast2 = new Beverage( 'beer' );
console.log ( breakfast );
console.log ( breakfast2 );

Now you can do this in ES6

function Beverage ( type = 'sparkling water' ){
  this.type = type;
}
var breakfast = new Beverage();
var breakfast2 = new Beverage( 'rocket fuel' );
console.log ( breakfast );
console.log ( breakfast2 );

Array.isArray()

If you have tried to confirm if something is an array with ES5, you may have found it frustrating:

var arr = [1,2,3];

console.log(typeof arr); //'object'

You could do

arr instanceof Array; // true

But now there is a new method!

Array.isArray(arr); //true

Argument Object

This isn't an ES6 thing, but should help expand your knowledge of arguments in JS.

The arguments object is an array-like object (but not fully an array, so you cannot always call array methods on it). It allows for a lot of flexibility in the number of arguments a function can take:

function sum (){
    var sum = 0;
    for(var i = 0; i < arguments.length; i++ ){
        sum += arguments[i];
    }
    return sum;
}

console.log( sum(1,2,3,4) ); //10

Spread and Rest Operators

Spread operator:

Spread Operator will take an array, and convert each element into its own value. It "spreads" the elements out into values

Let's look at Math.max() which takes any number of arguments and finds the maximum value

//Standard syntax
console.log(Math.max(1,5,-6)); // 5

• But what if you have an array of values?
• Converting each to a variable and then adding them as arguments is a pain, until now:

var z = [1,5,-2,8,-9,17,-22];
console.log(Math.max(...z)); // 17

Rest operator:

The rest operator gathers many values into an array. It's basically the opposite of spread.

function returnOnlyNums(...arrayParam){
    var nums = arrayParam.filter(arg => typeof arg === 'number');
    return nums;
}

console.log( returnOnlyNums(44, false, 'pizza', 45, {season: "winter"}, [1,2,3,4,5,], 2, 9) ); // [ 44, 45, 2, 9 ]

Template Literals (String Interpolation)

Template literals allow you to insert variables into strings with out having to use +

The old way:

var name = "Matt"
console.log ('Hello '+ name);

Now we can do this:

var name = "Matt"
console.log (`Hello ${name}.`);

You can even evaluate JavaScript within the Template Literal:

var name = "Matt";

function yell(value){
    return value.toUpperCase();
}

console.log (`Hello ${yell(`${name}`)}!`);

Multiple lines used to suck:

var adjective = 'awesome'
var template = 'I wrote a haiku\n'+
'This is so very ' + adjective + '\n'+
'Now I am finished';

console.log(template);

Now they're easy!

var adjective = 'awesome';
var template = `I wrote a haiku
This is so very ${adjective}
Now I am finished`;

console.log(template);

Object literal upgrades

Instead of:

var a = 8;
var b = 9;
var c = 10;

var es5obj = {
    a:a,
    b:b,
    c:c,
    d : function (){
        console.log(this.a, this.b, this.c);
    }

}
es5obj.d();

Now do:

var a = 8;
var b = 9;
var c = 10;

var es6obj = {
    a,
    b,
    c,
    d(){
        console.log(this.a, this.b, this.c);
    }
}
es6obj.d();

Destructuring

Destructuring pulls variables out of their "structure" (array or object)

Array Destructuring

You can assign values an array to variables easily:

var a, b;
[a,b] = [10,12]
console.log(a);
console.log(b);

You can also declare in the same line as assignment:

var [a,b] = [10,12]
console.log(a);
console.log(b);

You can ignore certain values:

var a, b;
[a,,b] = [1,2,3,4];
console.log(a);
console.log(b);

You can also use a rest operator to gather variables into an array after a certain point:

var a, b, rest;
[a,b,...rest] = [10, 20, 30, 40, 50];
console.log(a);
console.log(b);
console.log(rest);

You can set defaults as well:

var a, b;

[a=5, b=7] = [1];
console.log(a); // 1
console.log(b); // 7

Object Destructuring

Pull variables out of an object

var p, q;

{p, q} = {p: 42, q: true};

console.log(p); // 42
console.log(q); // true

You can also change the names of the variables:

var o = {p: 42, q: true};
var {p: foo, q: bar} = o;

console.log(foo); // 42
console.log(bar); // true

You can also do default values:

var {a = 10, b = 5} = {a: 3};

console.log(a); // 3
console.log(b); // 5

Use this for default options arguments:

function drawES2015Chart({size = 'big', cords = {x: 0, y: 0}, radius = 25} = {}) {
    console.log(size, cords, radius);
}

drawES2015Chart({
    cords: {x: 18, y: 30},
    radius: 30
});

You can pull values from a function argument that's an object:

var user = {
    id: 42,
    displayName: 'jdoe',
    fullName: {
        firstName: 'John',
        lastName: 'Doe'
    }
};

function userId({id}) {
    return id;
}

console.log('userId: ' + userId(user)); // "userId: 42"

If you have a property name which is not a valid variable name, you can reassign it:

const foo = { 'fizz-buzz': true }
const { 'fizz-buzz': fizzBuzz } = foo
console.log(fizzBuzz);

Lastly, you can compute a variable name:

let key = 'z';
let {[key]: foo} = {z: 'bar'};

console.log(foo); // "bar"

Swap values: File under Destructuring Assignment

If you wanted to swap the value of x and y with es5, you had to do

var x = true;
var y = false;
var temp;

temp = x;
x = y;
y = temp;
console.log(x,y);

The new way will return x and y to original values in this case

var x = true;
var y = false;

[x,y] = [y,x];

console.log(x,y);

Note

The reason this is special and new is because we have to remember that array values are normally passed by reference. It is also unusual to have an array that is not assigned to a variable

// Pass by reference
var a =  1;
var b =  2;

var originalArray = [a,b];
console.log('is `a` equal to orginalArray[0]:', a === originalArray[0]);//true

var newArray = originalArray;

//will reverse BOTH arrays (because it is actually two references to the same array)
newArray.reverse()

console.log('This is newArray.reverse():', newArray)
console.log('This is originalArray after newArray has been reversed', originalArray)
console.log('is `a` equal to orginalArray[0]:', a === originalArray[0]);//false

To make a duplicate array that is not passed by reference, you would have to do something like:

var a =  1;
var b =  2;

var anotherOriginalArray = [a,b];
console.log('is `a` equal to anotherOrginalArray[0]:', a === anotherOriginalArray[0]);//true

var trueNewArray = anotherOriginalArray.map(function(e){
    return e;
});

console.log('this is trueNewArray', trueNewArray);
trueNewArray.reverse();
console.log('\nThis is trueNewArray.reverse():', trueNewArray);
console.log('This is anotherOriginalArray after trueNewArray has been reversed', anotherOriginalArray);
console.log('is `a` equal to anotherOriginalArray[0]:', anotherOriginalArray[0] === a); //true